The recycling of post consumer plastics is a relatively recent phenomenon which is rapidly growing in importance due to growing popular support for recycling and the increased use of plastics in such consumer items as pop bottles and milk bottles. The support is shown in such programs as the widespread "blue box" curbside collection programs, which have been encouraged by greater popular concern for environmental issues, legislation and the increasing cost of waste disposal by land fill. The result is a rapidly growing supply of post consumer plastics for recycling.
The primary criterion for plastic recycling is the purity of the end product. Minute contamination of the recovered plastic can greatly reduce the value of the recovered plastic. Previous plastic recycling technology relies on "float-sink" technology, according to which the target plastic was separated from waste of different specific gravities by grinding the feed material and choosing a liquid of appropriate specific gravity to float off the waste and allow the target plastic to sink, or vice versa. However, this method did not produce an end product of sufficient purity. Recently, hydrocyclones, previously used in mineral separation and other industries, have been applied to plastic separation, and these devices have produced a good improvement over the old technology. For example, European Patent application no. 88112333 published Mar. 1, 1989 discloses a system for recovering plastics which uses a two-stage hydrocyclone step to separate the heavier PET plastics from lighter plastics and other waste. The underflow from the first hydrocyclone in this system feeds the second hydrocyclone and the underflow from the second hydrocyclone is dewatered, dried and sorted.